Integrated multiple-loop refrigeration process for gas liquefaction

ABSTRACT

Method gas liquefaction which comprises cooling a feed gas stream successively through at least two heat exchange zones, wherein cooling is provided by respective vaporizing refrigerants, and wherein the refrigerant in the coldest temperature range is only partially vaporized in the coldest heat exchange zone and then is vaporized in a further heat exchange zone at temperatures above the highest temperature of the coldest heat exchange zone to form a totally vaporized refrigerant. The totally vaporized refrigerant is compressed to yield a compressed refrigerant stream, and the entire compressed refrigerant stream is either (i) cooled by indirect heat exchange in the further heat exchange zone, thereby providing self-refrigeration for the recirculating refrigeration process, or (ii) cooled in a heat exchange zone preceding the coldest heat exchange zone by indirect heat exchange with a respective vaporizing refrigerant and then further cooled the in the further heat exchange zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 10/780,613 filed onFeb. 19, 2004, which is a continuation-in-part of U.S. Ser. No.10/391,390 filed on Mar. 18,2003, now U.S. Pat. No. 6,742,357. Both ofthese applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Multiple-loop refrigeration systems are widely used for the liquefactionof gases at low temperatures. In the liquefaction of natural gas, forexample, two or three closed-loop refrigeration systems may beintegrated to provide refrigeration in successively lower temperatureranges to cool and liquefy the feed gas. Typically, at least one ofthese closed-loop refrigeration systems uses a multi-component or mixedrefrigerant which provides refrigeration in a selected temperature rangeas the liquid mixed refrigerant vaporizes and cools the feed gas byindirect heat transfer. Systems using two mixed refrigerant systems arewell-known; in some applications, a third refrigerant system using apure component refrigerant such as propane provides initial cooling ofthe feed gas. This third refrigerant system also may be used to providea portion of the cooling to condense one or both of the mixedrefrigerants after compression. Refrigeration in the lowest temperaturerange may be provided by a gas expander loop that is integrated with amixed refrigerant loop operating in a higher temperature range.

In a typical multi-loop mixed refrigerant process for liquefying naturalgas, the low level or coldest refrigeration loop provides refrigerationby vaporization in a temperature range of −30 to −165° C. to providefinal liquefaction and optional subcooling of cooled feed gas. Therefrigerant is completely vaporized in the coldest temperature range andmay be returned directly to the refrigerant compressor, for example, asdescribed in representative U.S. Pat. Nos. 6,119,479 and 6,253,574 B1.Alternatively, the completely vaporized refrigerant may warmed beforecompression to provide precooling of the feed gas as described in U.S.Pat. Nos. 4,274,849 and 4,755,200 or for cooling of refrigerant streamsas described in Australian Patent AU-A-43943/85. A common characteristicfeature of these typical liquefaction processes is that the refrigerantin the low level or coldest refrigeration loop is completely vaporizedwhile providing refrigeration in the lowest temperature range. Anyadditional refrigeration provided by the refrigerant prior tocompression thus is effected by the transfer of sensible heat from thevaporized refrigerant to other process streams.

In known liquefaction processes that use three integrated closed-looprefrigeration systems, the size of the process equipment in the third orlowest temperature refrigeration system may be smaller relative to thetwo warmer refrigeration systems. As the process liquefaction capacityis increased, the sizes of the compression and heat exchange equipmentin the two warmer systems will reach the maximum sizes available fromequipment vendors, while the sizes of the corresponding equipment in thelowest temperature refrigeration system will be smaller than the maximumsizes. In order to further increase the production capacity of thisliquefaction process, parallel trains would be needed because ofcompression and/or heat exchanger size limitations in the two warmerrefrigeration systems. It would be desirable to increase the maximumproduction capacity of this liquefaction process at the limits ofavailable compressor and heat exchanger sizes, thereby allowing the useof larger single-train liquefaction processes.

BRIEF SUMMARY OF THE INVENTION

This need is addressed by embodiments of the present invention, whichrelate to integrated refrigeration systems having increased productioncapacity without requiring duplicate parallel equipment for the warmerrefrigeration systems. One embodiment of the invention provides a methodfor liquefying a gas which comprises cooling a feed gas streamsuccessively through at least two heat exchange zones (310, 311, 312;353) at respective temperature ranges to provide a liquefied product(13), wherein refrigeration for cooling the feed gas stream in thetemperature ranges is provided by respective vaporizing refrigerants(117, 213, 315), wherein the refrigerant (315) in the coldesttemperature range is only partially vaporized in the coldest heatexchange zone (312) to form a partially vaporized refrigerant (316), andwherein the refrigerant is recirculated in a recirculating refrigerationprocess that comprises further vaporizing the partially vaporizedrefrigerant (316) in a further heat exchange zone (317, 355) attemperatures above the highest temperature of the coldest heat exchangezone (312) to form a totally vaporized refrigerant (318, 348),compressing (319, 324; 349) the totally vaporized refrigerant (318, 348)to yield a compressed refrigerant stream, and cooling the compressedrefrigerant stream to provide a coldest refrigerant (315), characterizedin that the entire compressed refrigerant stream is cooled by either

(i) cooling the entire compressed refrigerant stream (328) in thefurther heat exchange zone (317) by indirect heat exchange with thefurther vaporizing partially vaporized refrigerant (316) to provide acooled refrigerant stream (329), thereby providing self-refrigerationfor the recirculating refrigeration process, and then by further cooling(312) the cooled refrigerant stream (329) to provide the coldestrefrigerant (315), or

(ii) cooling the entire compressed refrigerant stream (351) in a heatexchange zone (353) preceding the coldest heat exchange zone (312) byindirect heat exchange (352) with a respective vaporizing refrigerant(117), further cooling the refrigerant in the further heat exchange zone(355) by indirect heat exchange with the partially vaporized refrigerant(316) to provide a cooled refrigerant stream (329), and then furthercooling (312) the cooled refrigerant stream (329) to provide the coldestrefrigerant (315).

The feed gas stream (1) may be natural gas. The refrigerant (315, 316,318, 328, 329) in the recirculating refrigeration process may be amulticomponent mixture comprising nitrogen, i-pentane, and n-pentanewith the molar ratio of i-pentane to n-pentane in the refrigerant (315,316, 318, 328, 329) being greater than one, and wherein the i-pentaneand n-pentane are obtained from the feed gas stream (1) and the molarratio of i-pentane to n-pentane in the refrigerant (315, 316, 318, 328,329) is greater than the molar ratio of i-pentane to n-pentane in thefeed gas stream (1). The refrigerant (315, 316, 318, 328, 329) in therecirculating refrigeration process may be a multicomponent mixturecomprising nitrogen, i-pentane, and one or more hydrocarbons having fourcarbon atoms, the i-pentane and the one or more hydrocarbons having fourcarbon atoms being obtained from the feed gas stream (1), and the molarratio of i-pentane to n-pentane in the refrigerant (315, 316, 318, 328,329) being greater than one, wherein the i-pentane and n-pentane areobtained from the feed gas stream (1) and the molar ratio of i-pentaneto the one or more hydrocarbons having four carbon atoms in therefrigerant (315, 316, 318, 328, 329) is greater than the molar ratio ofi-pentane to the one or more hydrocarbons having four carbon atoms inthe feed gas stream (1).

The refrigerant (315, 316, 318, 328, 329) in the recirculatingrefrigeration process may comprise (in mole %) 5-15% nitrogen, 30-60%methane, 10-30% ethane, 0-10% propane, and 5-15% i-pentane.

The further cooling of the cooled refrigerant stream (329) may beeffected by indirect heat exchange with the coldest refrigerant (315)vaporizing in the coldest heat exchange zone (312). Prior tovaporization to cool the compressed refrigerant stream (328, 339), acooled reduced-pressure liquid refrigerant (345) may be reduced inpressure and combined with the partially vaporized refrigerant (316) toprovide a combined two-phase refrigerant (347) that is vaporized to coolthe compressed refrigerant stream (328; 339), the compressed refrigerantvapor (330) may be cooled (332) to provide a partially-condensedrefrigerant, the partially condensed refrigerant may be separated (333)into a refrigerant vapor stream (334) and a refrigerant liquid stream(335), the refrigerated vapor stream (334) may be compressed (336) andcooled (337) to form a partially condensed stream and thepartially-condensed stream may be separated (338) into the compressedrefrigerant vapor (328, 339) and a liquid stream (340), the pressure ofthe liquid refrigerant may be reduced (341) to provide areduced-pressure liquid refrigerant (342), and the reduced-pressureliquid refrigerant (342) may be combined with the refrigerant liquidstream (343) and subcooled by indirect heat exchange (344) with thecombined two-phase refrigerant (347) to provide the cooledreduced-pressure liquid refrigerant (345) for combining with thepartially vaporized refrigerant (316).

Another embodiment of the invention includes a system for liquefying agas stream (1) which comprises:

at least two heat exchange zones (310, 311 312; 353) adapted for coolingthe gas stream (1) successively through respective temperature ranges toprovide a liquefied product (13) and

respective refrigeration systems for providing respective refrigerantsin respective refrigerant lines (117, 213, 315) to the heat exchangezones (310, 311, 312; 353),

wherein the coldest heat exchange zone (312) is adapted to onlypartially vaporize the respective (i.e., coldest) refrigerant (315),

wherein the refrigeration system providing the coldest refrigerant is arecirculating refrigeration system comprising:

a further heat exchange zone (317, 355) adapted to further vaporize theresultant partially vaporized refrigerant at temperatures above thehighest temperature of the coldest heat exchange zone (312),

compression means (319, 324; 349) for compressing the vaporizedrefrigerant to provide the compressed refrigerant stream,

piping means (318, 348) to provide vaporized refrigerant from thefurther heat exchange zone (317, 355) to the compression means (319,324; 349),

means to provide compressed refrigerant (328, 354) to the further heatexchange zone (317, 355),

piping means to convey a cooled compressed refrigerant from the furtherheat exchange zone (317; 355) to the coldest heat exchange zone (312),and

means to further cool (356) the cooled compressed refrigerant to providea cooled compressed refrigerant (313),

characterized in that the means to provide compressed refrigerant (328;354) to the further heat exchange zone (317, 355) comprises either

(i) piping means (329) to convey the entire compressed refrigerantstream (328) to the further heat exchange zone (317), wherein thefurther heat exchange zone (317) is adapted to provideself-refrigeration for the recirculating refrigeration system, or

(ii) piping means to convey the entire compressed refrigerant stream(351) to a heat exchange zone (353) that precedes the coldest heatexchange zone (312), wherein the heat exchange zone (353) is adapted tocool the entire compressed refrigerant stream (351) by indirect heatexchange (352), and piping means to convey an intermediate cooledcompressed refrigerant (354) to the further heat exchange zone (355).

In the system of this embodiment, the means to further cool (356) thecooled compressed refrigerant to provide a condensed refrigerant maycomprise the coldest heat exchange zone (312) and the system may furthercomprise pressure reduction means (314) to reduce the pressure of thecondensed refrigerant to provide the refrigerant to the refrigerant line(315) for the coldest heat exchange zone (312).

The further heat exchange zone (344) may include means for subcooling arefrigerant liquid to provide a subcooled refrigerant liquid and thecoldest refrigeration system may comprise pressure reduction means (346)for reducing the pressure of the subcooled refrigerant liquid to providea reduced-pressure refrigerant, piping means (347) for combining thereduced-pressure refrigerant with the partially vaporized refrigerantfrom the coldest heat exchange zone (312) to provide a combinedvaporizing refrigerant stream to the further heat exchange zone (344),and piping means (330) to feed a combined vaporized refrigerant streamto the compression means.

The compression means for compressing the vaporized refrigerant (330)from the further heat exchange zone (344) may comprise:

a first stage compressor (331),

an intercooler (332) adapted to cool and partially condense theresultant first compressed refrigerant stream from first stagecompressor (331) to yield a partially-condensed first refrigerantstream,

a first separator (333) to separate the partially condensed firstcompressed refrigerant stream into a first vapor refrigerant stream anda first liquid refrigerant stream,

a second stage compressor (336) to compress the first vapor refrigerantstream to provide a compressed vapor refrigerant stream,

an aftercooler (337) to cool the compressed vapor refrigerant stream toprovide a cooled two-phase refrigerant stream,

a second separator (338) to provide a second liquid refrigerant stream(340) and the and the compressed refrigerant to piping means (339) forfeed to the further heat exchange zone (344),

pressure reduction means (341) to reduce the pressure of the secondliquid refrigerant stream to provide a reduced-pressure secondrefrigerant stream, and

piping means (335, 342, 343) to combine the reduced-pressure secondrefrigerant stream and the first liquid refrigerant stream to providethe refrigerant liquid to the further heat exchange zone (344).

An alternative embodiment of the invention relates to a system forliquefying a gas stream through at least two heat exchange zones, eachof which is cooled by a respective refrigeration system, wherein thecoldest of the respective refrigeration systems comprises:

piping means (348) to provide the vaporized refrigerant from the furtherheat exchange zone (355) to the compression means (349) for compressinga vaporized third refrigerant (348) to provide a compressed refrigerant,

cooling means (352) in a heat exchange zone preceding the coldest heatexchange zone (312) for cooling the compressed refrigerant (351) byindirect heat exchange with the respective refrigerant (117) vaporizingin the heat exchange zone (353) to provide a cooled compressedrefrigerant,

piping means (354) to provide the cooled compressed refrigerant to thefurther heat exchange zone (355) to further cool the cooled compressedrefrigerant by indirect heat exchange with the vaporizing refrigerantfrom the coldest heat exchange zone (312) to provide a condensedrefrigerant (329) and the vaporized third refrigerant (348), and

pressure reduction means (314) to reduce the pressure of the condensedrefrigerant to provide the refrigerant to the refrigerant line (315) tothe coldest refrigerant zone (312).

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a gas liquefaction andrefrigeration system according to the prior art.

FIG. 2 is a schematic flow diagram of a gas liquefaction andrefrigeration system according to an exemplary embodiment of the presentinvention.

FIG. 3 is a schematic flow diagram of a gas liquefaction andrefrigeration system according to an alternative exemplary embodiment ofthe present invention.

FIG. 4 is a schematic flow diagram of a gas liquefaction andrefrigeration system according to another exemplary embodiment of thepresent invention.

FIG. 5 is a schematic flow diagram of a gas liquefaction andrefrigeration system according to another alternative exemplaryembodiment of the present invention.

FIG. 6 is a schematic flow diagram of a gas liquefaction andrefrigeration system according to another alternative exemplaryembodiment of the present invention utilizing two pressure levels forthe vaporization of the coldest refrigerant.

FIG. 7 is a schematic flow diagram of a gas liquefaction andrefrigeration system according to another alternative exemplaryembodiment of the present invention utilizing phase separation of therefrigerant used in the coldest temperature range.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention described herein relate to improvedrefrigeration processes for gas liquefaction utilizing three closed-looprefrigeration systems that cool a feed stream through three temperatureranges at successively-decreasing temperatures. These embodiments aredirected towards improvements to the refrigeration system that providesrefrigeration in the lowest of these temperature ranges, wherein thesizes of the compressor and heat exchange equipment used in therefrigeration system in the lowest temperature range are increasedrelative to the sizes of the compressors and heat exchangers in therefrigeration systems used in the higher temperature ranges. The termrefrigeration as used herein means the indirect transfer of heat attemperatures below ambient from a fluid stream to a refrigerant. Arefrigerant is a pure or mixed fluid which absorbs heat from anotherstream by indirect heat exchange with that stream.

A schematic flow diagram of a representative prior art liquefactionprocess is given in FIG. 1. Feed gas in line 1, for example, natural gashaving been pretreated to remove water and other easily condensibleimpurities, is cooled through a first temperature range by indirect heatexchange with a first vaporizing refrigerant in first heat exchange zoneor heat exchanger 3. The refrigerant may be a pure component refrigerantsuch as propane or alternatively may be a multi-component refrigerantcomprising two or more light hydrocarbons selected from ethane,ethylene, propane, propylene, butane, and isobutane.

The cooled feed in line 5 is further cooled through a second temperaturerange by indirect heat exchange with a second vaporizing refrigerant insecond heat exchange zone or heat exchanger 7. The further cooled feedin line 9 is still further cooled and liquefied through a thirdtemperature range by indirect heat exchange with a third vaporizingrefrigerant in third heat exchange zone or heat exchanger 11. Therefrigerant typically is a multi-component refrigerant comprising two ormore refrigerant components selected from methane, ethane, ethylene,propane, and propylene. Final liquefied product in line 13 may bereduced in pressure across expansion valve 15 to yield final liquidproduct in line 17.

Refrigeration for this process typically is provided by three nested orcascaded refrigeration systems. The first refrigeration system operatesby supplying vapor refrigerant in line 101 to first compressor stage103, wherein the gas is compressed to 2 to 4 bara (all pressures statedherein are absolute pressures), cooled in aftercooler 105, furthercompressed to 6 to 10 bara in second compressor 107, and cooled inaftercooler 109 to provide a compressed refrigerant at ambienttemperature in line 111. The compressed refrigerant is further cooledand at least partially condensed in heat exchange passages in first heatexchange zone or heat exchanger 3. The partially or fully condensedrefrigerant in line 113 is reduced in pressure across throttling valve115 to provide reduced pressure refrigerant in line 117, and thisrefrigerant vaporizes in separate heat exchange passages to provide therefrigeration in first heat exchange zone or heat exchanger 3. Vaporizedrefrigerant in line 101 is compressed as described above.

The second refrigeration system operates by supplying vapor refrigerantin line 201 to compressor 203, wherein the gas is compressed to 10 to 20bara and cooled in aftercooler 205 to approximately ambient temperature.The compressed refrigerant in line 207 is further cooled and at leastpartially condensed in heat exchange passages in first heat exchangezone or heat exchanger 3 and second heat exchange zone or heat exchanger7. The partially or fully condensed refrigerant in line 209 is reducedin pressure across throttling valve 211 to provide reduced pressurerefrigerant in line 213, and this refrigerant vaporizes in separate heatexchange passages to provide the refrigeration in second heat exchangezone or heat exchanger 7. Vaporized refrigerant in line 201 iscompressed as described above.

The third refrigeration system operates by supplying vapor refrigerantin line 301 to compressor 302, wherein the gas is compressed to 35 to 60bara and cooled in aftercooler 303 to approximately ambient temperature.The compressed refrigerant in line 304 is further cooled and at leastpartially condensed in heat exchange passages in first heat exchangezone or heat exchanger 3, second heat exchange zone or heat exchanger 7,and third heat exchange zone or heat exchanger 11. The partially orfully condensed refrigerant in line 305 is reduced in pressure acrossthrottling valve 307 to provide reduced pressure refrigerant in line309, and this refrigerant vaporizes in separate heat exchange passagesto provide the refrigeration in third heat exchange zone or heatexchanger 11. Vaporized refrigerant in line 301 is compressed asdescribed above. The use of the third refrigeration loop including heatexchanger 11 and compressor 302 provides a portion of the totalrefrigeration duty needed to liquefy the feed gas and reduces therefrigeration duties and sizes of the first and second refrigerationsystems.

Known modifications or alternatives to the prior art process using threerefrigeration loops of FIG. 1 are possible. For example, the firstrefrigeration loop may utilize cascade refrigeration in which therefrigerant is vaporized at three different pressures with the vaporizedrefrigerant returning to different stages in a multi-stage compressor.The second refrigeration loop may vaporize refrigerant at two differentpressures through two separate sets of heat exchange passages in heatexchanger 7 and return each vaporized refrigerant stream to two separatecompressor stages.

In another modification, the third refrigeration loop may vaporizerefrigerant at two different pressures through two separate sets of heatexchange passages in heat exchanger 11 and return each vaporizedrefrigerant stream to two separate compressor stages. Vaporizedrefrigerant in line 301 prior to compressor 302 may be used in aseparate heat exchanger to provide cooling for a portion of secondrefrigerant stream 215 and for a portion of compressed refrigerant inline 304.

In another known process with three refrigeration loops, vaporizingrefrigerant in the first refrigeration loop is used to precool the feedgas; the first refrigeration loop compressor discharge is cooled andcondensed by a portion of the vaporizing refrigerant from the secondrefrigeration loop. Vaporized refrigerant in the third refrigerationloop from the third heat exchanger prior to compression is used tofurther precool the feed gas. This further precooled feed gas then iscooled and condensed in the third heat exchanger. The secondrefrigeration loop cools and condenses the compressed third refrigerant.

A common characteristic feature of these known liquefaction processes isthat the refrigerant in the third refrigeration loop, i.e., the lowlevel or coldest refrigeration loop, is completely vaporized whileproviding refrigeration in the lowest temperature range. Any additionalrefrigeration provided by the refrigerant prior to compression iseffected only by the transfer of sensible heat from the vaporizedrefrigerant to other process streams.

In the several embodiments of the present invention, the condensedrefrigerant of the third or coldest refrigeration loop is only partiallyvaporized in the third heat exchange zone or heat exchanger in the thirdor lowest temperature range. The partially vaporized refrigerant fromthe third heat exchange zone or heat exchanger is further vaporized attemperatures above the lowest temperature in the second temperaturerange. This is illustrated by a first exemplary embodiment of theinvention shown in FIG. 2. Feed gas in line 1, for example, natural gashaving been pretreated to remove water and other condensible impurities,is cooled through a first temperature range by indirect heat exchangewith a first vaporizing refrigerant in first heat exchange zone or heatexchanger 310. The refrigerant may be a multi-component refrigerantcomprising, for example, two or more light hydrocarbons selected fromethane, ethylene, propane, butane, n-pentane, and i-pentane (i.e.,2-methyl butane). Alternatively, the refrigerant may be a singlecomponent such as propane. The upper temperature of the firsttemperature range may be ambient temperature and the lower temperaturein the first temperature range may be between about −35° C. and about−55° C. The specific refrigerant composition may be selected to achievea desired lower temperature in the first temperature range.

The cooled feed in line 5 is further cooled through a second temperaturerange by indirect heat exchange with a second vaporizing refrigerant insecond heat exchange zone or heat exchanger 311 to a temperature betweenabout −40° C. and about −100° C. The refrigerant typically is amulti-component refrigerant and may comprise, for example, two or morecomponents selected from methane, ethane, ethylene, and propane. Thespecific refrigerant composition may be selected to achieve a desiredlower temperature in the second temperature range.

The further cooled feed in line 9 is still further cooled and liquefiedthrough a third temperature range, reaching a lower temperature betweenabout −85° C. and about −160° C., by indirect heat exchange with a thirdvaporizing refrigerant in third heat exchange zone or heat exchanger317. This refrigerant is a multi-component refrigerant and may comprise,for example, two or more components selected from methane, ethane,ethylene, propane, propylene, one or more hydrocarbons having fourcarbon atoms, n-pentane, i-pentane (i.e., 2-methyl butane), andnitrogen. In this refrigerant, i-pentane is a preferred (but notrequired) component. The specific refrigerant composition may beselected to achieve a desired lower temperature in the third temperaturerange. Final liquefied product in line 13 may be reduced in pressureacross expansion valve 15 to yield final liquid product in line 17.

The first temperature range may be defined by a first temperature and asecond temperature, and the first temperature may be ambienttemperature. The second temperature range may be defined by the secondtemperature and a third temperature, and the third temperature range maybe defined by the third temperature and a fourth temperature. The firsttemperature range is the highest or warmest temperature range and thethird temperature range is the lowest or coldest temperature range. Thefirst temperature is the highest temperature and the fourth temperatureis the lowest temperature.

Refrigeration for this process may be provided by three nested orcascaded refrigeration systems. The first refrigeration system may besimilar to the first refrigeration system as described above withreference to FIG. 1, and may operate by supplying vapor refrigerant inline 101 to first compressor stage 103, wherein the gas is compressed to2 to 4 bara, cooled in aftercooler 105, further compressed to 6 to 10bara in second compressor 107, and cooled in aftercooler 109 to providea compressed refrigerant at ambient temperature in line 111. Thecompressed refrigerant is further cooled and at least partiallycondensed in heat exchange passages in first heat exchange zone or heatexchanger 310. The partially or fully condensed refrigerant in line 113is reduced in pressure across throttling valve 115 to provide reducedpressure refrigerant in line 117, and this refrigerant vaporizes inseparate heat exchange passages to provide the refrigeration in firstheat exchange zone or heat exchanger 3. Vaporized refrigerant in line101 is compressed as described above.

The second refrigeration system may be similar to the firstrefrigeration system as described above with reference to FIG. 1, andmay operate by supplying vapor refrigerant in line 201 to compressor203, wherein the gas is compressed to 10 to 20 bara and cooled inaftercooler 205 to approximately ambient temperature. The compressedrefrigerant in line 207 is further cooled and at least partiallycondensed in heat exchange passages in first heat exchange zone or heatexchanger 310 and second heat exchange zone or heat exchanger 311. Thepartially or fully condensed refrigerant in line 209 is reduced inpressure across throttling valve 211 to provide reduced pressurerefrigerant in line 213, and this refrigerant vaporizes in separate heatexchange passages to provide the refrigeration in second heat exchangezone or heat exchanger 311. Vaporized refrigerant in line 201 iscompressed as described above.

The third refrigeration system of this embodiment departs from the priorart third refrigeration system described earlier and operatesindependently of the first and second refrigeration systems. In thisthird refrigeration system, condensed refrigerant in line 313 is reducedin pressure across throttling valve 314 and reduced-pressure condensedrefrigerant from line 315 is partially vaporized in third heat exchangezone or heat exchanger 312 to provide refrigeration therein.Partially-vaporized refrigerant flows through line 316 and vaporizescompletely in fourth heat exchange zone or heat exchanger 317 to providerefrigeration therein. Vaporized refrigerant in line 318, typically atnear ambient temperature and a pressure of 2 to 10 bara is compressed infirst compressor 319, cooled and partially condensed in intercooler 320,and separated in separator 321 to provide a vapor stream in line 322 anda liquid stream in line 323.

The vapor stream in line 322 is further compressed to a pressure of 30to 70 bara in compressor 324, the liquid stream in line 323 ispressurized by pump 325 to the same pressure, the two pressurizedstreams are combined to provide two-phase refrigerant stream 326, whichis further cooled in aftercooler 327. Partially or fully condensedrefrigerant in line 328 is further cooled in fourth heat exchange zoneor heat exchanger 317 to provide cooled refrigerant in line 329. Thecooled refrigerant in line 329 is further cooled in flow passages 356 ofthird heat exchange zone or heat exchanger 312 to yield refrigerant 313described above.

The mixed refrigerant used in the third refrigerant system containsselected components and compositions that allow the refrigerant tovaporize over a broad temperature range. The criteria for selectingthese components and the temperature range over which the refrigerantvaporizes are different than the criteria for selecting the mixedrefrigerants typically used in the third or low level refrigeration loopof three-loop liquefaction systems known in the art. The mixedrefrigerant in the third loop of the present invention should be capableof vaporizing in the third temperature range (i.e., in third heatexchange zone or heat exchanger 312) as well as at temperatures abovethe lowest temperature in the second temperature range (i.e., above thelowest temperature in second heat exchange zone or heat exchanger 311).Depending on the refrigerant composition and pressure, vaporization maybe possible and desirable at temperatures above the highest temperaturein the second temperature range.

Typical compositions (in mole %) of the refrigerant used in the thirdloop may include 5-15% nitrogen, 30-60% methane, 10-30% ethane, 0-10%propane, and 5-15% i-pentane. One or more hydrocarbons having fourcarbon atoms may be present in the refrigerant, but preferably the totalconcentration of the one or more hydrocarbons having four carbon atomsis lower than the concentration of i-pentane. The molar ratio ofi-pentane to the one or more hydrocarbons having four carbon atoms inthe refrigerant typically is greater than one and may be greater than1.5. Normal pentane (n-pentane) also may be present in the refrigerant,preferably at lower concentrations than i-pentane.

The refrigeration components for use in the third refrigeration loop maybe provided from hydrocarbon liquids heavier than methane that arecondensed by initial cooling of a natural gas feed. These condensednatural gas liquids (NGLs) may be recovered and fractionated by knownmethods to obtain the individual components for use in the preferredmixed refrigerant. When the natural gas feed contains both n-pentane andi-pentane, for example, and when these components are recovered fromNGLs by distillation for use in the refrigerant in the third refrigerantloop, the molar ratio of i-pentane to n-pentane in the refrigerant maybe greater than the molar ratio of i-pentane to n-pentane in the feedgas. Preferably, the molar ratio of i-pentane to n-pentane in therefrigerant is greater than twice the molar ratio of i-pentane ton-pentane in the feed gas. i-pentane is preferred over n-pentane for usein this refrigerant because i-pentane has a lower freezing point thann-pentane, which allows the refrigerant to be used at lowertemperatures.

When the natural gas feed contains both i-pentane and one or morehydrocarbons having four carbon atoms, and when these components arerecovered from NGLs by distillation for use in the refrigerant in thethird refrigerant loop, the molar ratio of i-pentane to the one or morehydrocarbons having four carbon atoms in the refrigerant may be greaterthan the molar ratio of i-pentane to the one hydrocarbons having fourcarbon atoms in the feed gas.

The third refrigeration loop in this embodiment is self-refrigerated andis independent of the first and second refrigeration loops. In contrastwith the process of FIG. 1, compressed refrigerant in the thirdrefrigeration loop of FIG. 2 is not cooled in the first and second heatexchange zones by the first and second refrigeration loops. This unloadsthe first and second refrigeration loops, and thus reduces the sizes ofthe first and second heat exchange zones and the compression equipmentin the first and second refrigeration loops compared to the process ofFIG. 1. This is particularly beneficial when the process of FIG. 2 isused in a liquefaction system designed for a very large productthroughput. When the sizes of the compression and heat exchangeequipment in the first and second refrigeration loops reach the maximumsizes available from equipment vendors, a higher production rate can beachieved with the process of FIG. 2 than with the process of FIG. 1.

Variations to the process embodiment of FIG. 2 are possible. Forexample, one stage or more than two stages of compression may be used asrequired, which would form multiple liquid streams for pumping inconjunction with the vapor compression stages. In another variation, therefrigerant composition and pressures in the compression system may besuch that interstage condensation does not occur and vapor/liquidseparation is not required.

In an alternative embodiment of the process of FIG. 2, the secondrefrigeration system is not required, and heat exchanger 311, valve 211,compressor 203, cooler 205, and the associated piping are not used. Inthis alternative, heat exchange zone or heat exchanger 310 would notinclude passages for cooling refrigerant supplied via line 207. Theprocess in this embodiment therefore comprises cooling the feed gas inline 1 successively through first and second temperature ranges toprovide a liquefied product in line 13, wherein refrigeration forcooling the gas stream is provided by a first refrigerant in line 117vaporizing in the first temperature range and a second refrigerant inline 315 vaporizing in the second temperature range and furthervaporizing at temperatures above a lowest temperature in the firsttemperature range. Thus the temperature ranges in which the first andsecond refrigerants vaporize overlap. In this alternative embodiment,the first refrigerant may be propane and the second refrigerant may be amulti-component refrigerant. In another version of this embodiment, bothrefrigerants may be selected multi-component refrigerants.

An alternative embodiment of the exemplary process of FIG. 2 isillustrated in FIG. 3. In this alternative, the first refrigeration loopof FIG. 2 (compressors 103 and 107, coolers 105 and 109, and throttlingvalve 115) is replaced by a single-component cascade refrigerationsystem. Propane may be used as the single refrigerant in the firstrefrigeration loop. In this embodiment, the second and thirdrefrigeration loops remain unchanged from the embodiment of FIG. 2.

Multi-stage compressor 119 and aftercooler 121 are operated to providecompressed refrigerant in line 123 at near ambient temperature and apressure in the range of 10 to 15 bara. The compressed refrigerantinline 123 is reduced in pressure across throttling valve 125 and thereduced-pressure refrigerant in line 127 is partially vaporized in heatexchange zone or heat exchanger 129 to provide refrigeration therein andyield a two-phase refrigerant in line 131. This two-phase refrigerant isseparated in separator 133 to provide vapor in line 135, which vapor isreturned to a lower pressure stage suction of compressor 119, and liquidin line 137. This liquid is reduced in pressure across throttling valve139 and is partially vaporized in heat exchanger 129 to providerefrigeration therein. Two-phase refrigerant in line 141 is separated inseparator 143 to yield vapor in line 145, which vapor is returned to anintermediate stage suction of compressor 119, and liquid in line 147.This liquid is reduced in pressure across throttling valve 149 and thereduced-pressure refrigerant is vaporized in heat exchanger 129 toprovide additional refrigeration therein. Vapor in line 151 is returnedto the inlet of compressor 119.

Another alternative to the exemplary embodiment of FIG. 2 is illustratedin FIG. 4. In this embodiment, a modified third refrigeration loop isused wherein liquid formed in the compression step is combined withpartially vaporized liquid from the third heat exchanger and thecombined stream provides the refrigeration to cool the compressedrefrigerant vapor. Vaporized refrigerant in line 330 is compressed incompressor 331 to 2 to 10 bara, cooled and partially condensed inaftercooler 332, and separated in separator 333 to provide vapor in line334 and liquid in line 335. The vapor in line 334 is further compressedin compressor 336 to 6 to 20 bara, cooled and partially condensed inaftercooler 337, and separated in separator 338 to provide vapor in line339 and liquid in line 340.

The liquid in line 340 is reduced in pressure across throttling valve341, the reduced-pressure liquid in line 342 is combined with the liquidfrom line 335, and the combined liquid in line 343 is subcooled infourth heat exchange zone or heat exchanger 344 to yield a subcooledliquid refrigerant in line 345. This subcooled refrigerant is reduced inpressure across throttling valve 346 and combined withpartially-vaporized refrigerant in line 316 from third heat exchangezone or heat exchanger 312. The combined refrigerant in line 347 isvaporized in heat exchanger 344 to provide refrigeration therein andyield refrigerant vapor in line 330. Cooled refrigerant in line 329 isfurther cooled and at least partially liquefied in third heat exchangezone or heat exchanger 312, reduced in pressure across throttling valve314 to provide reduced-pressure refrigerant in line 315, wherein thereduced-pressure refrigerant is partially vaporized in heat exchanger312 to provide refrigeration therein as described above.Partially-vaporized refrigerant in line 316 returns to heat exchanger344 as described above.

Another alternative to the exemplary embodiment of FIG. 2 is illustratedin FIG. 5. In this exemplary embodiment, a modified third refrigerationloop is used wherein the refrigerant is compressed at sub-ambienttemperatures and a portion of the cooling of the compressed refrigerantis provided by the first refrigeration loop. Referring to FIG. 5,refrigerant vapor in line 348 at a temperature in the range of 0 to −90°C. is compressed in compressor 349 to 10 to 20 bara and cooled inaftercooler 350 to ambient temperature. Cooled compressed refrigerant inline 351 is further cooled in flow passages 352 of first heat exchangezone or heat exchanger 352, wherein refrigeration is provided by thefirst refrigeration loop as earlier described.

Cooled refrigerant in line 354 is further cooled in fourth heat exchangezone or heat exchanger 355 to provide a further cooled refrigerant inline 329. Cooled refrigerant in line 329 is further cooled and at leastpartially liquefied in third heat exchange zone or heat exchanger 312,reduced in pressure across throttling valve 314 to providereduced-pressure refrigerant in line 315, wherein the reduced-pressurerefrigerant is partially vaporized in heat exchanger 312 to providerefrigeration therein as described above. Partially-vaporizedrefrigerant in line 316 returns to heat exchanger 354 as describedabove.

In this alternative embodiment, mixed refrigerant in line 348 is at atemperature in the range of 0 to −90° C. at the inlet to compressor 349.The use of cold compression in compressor 349 contrasts with theembodiments of FIGS. 2, 3, and 4, wherein the refrigerant vapor entersthe compressor inlet at approximately ambient temperature. The mixedrefrigerant in the embodiment of FIG. 5 is lighter than the refrigerantin the embodiment of FIG. 2; preferably, the mixed refrigerant of FIG. 5contains no components heavier than propane.

Any of the embodiments of FIGS. 1 to 5 may be installed as a retrofit toan existing two-loop dual mixed refrigerant liquefaction plant ortwo-loop propane-mixed refrigerant natural gas liquefaction plant.

Another alternative to the exemplary embodiment of FIG. 2 is illustratedin FIG. 6. In this exemplary embodiment, the third refrigeration loopthat provides refrigeration to cold heat exchanger 312 is modified sothat the refrigerant is totally vaporized and compressed at sub-ambienttemperatures. A portion of the cooling of the compressed refrigerant isprovided by autorefrigeration in heat exchanger 357 at temperaturesabove the highest temperature of third heat exchange zone or heatexchanger 312 and above the lowest temperature of the feed stream inheat exchanger 311. The pressure of the vaporizing refrigerant in heatexchanger 357 is higher than the pressure of the vaporizing refrigerantin cold heat exchanger 312. In the embodiment of FIG. 6, the feed streamin line 9 being liquefied is cooled to its final lowest temperature inthis cold heat exchanger to provide liquid product in line 13, which maybe reduced in pressure to provide a reduced-pressure product in line 17.The refrigeration for this final cooling is provided by cooling therefrigerant in line 329 to provide cooled liquid refrigerant in line313, reducing the pressure across throttling valve 314 to yieldreduced-pressure refrigerant in line 315, and totally vaporizing thisrefrigerant to provide the refrigeration in heat exchanger 312.

The vaporized refrigerant in line 316 is compressed to a pressure in therange of 3 to 25 bara in first compressor 359 and the compressed streamin line 361 is cooled to near ambient temperature in cooler 363 toprovide intermediate compressed gas in line 365. The intermediatecompressed gas is combined with a vaporized auxiliary refrigerant streamin line 367 (described below) and the combined refrigerant stream isfurther compressed as described in the embodiments of FIGS. 2 and 3. Thecombined refrigerant stream, typically at near ambient temperature and apressure of 20 to 50 bara, is compressed in second compressor 319,cooled and partially condensed in intercooler 320, and separated inseparator 321 to provide a vapor stream in line 322 and a liquid streamin line 323.

The vapor stream in line 322 is further compressed to a pressure of 30to 70 bara in compressor 327, the liquid stream from separator 321 ispressurized by pump 325 to the same pressure, the two pressurizedstreams are combined to provide two-phase refrigerant stream 326, whichis further cooled in aftercooler 327 by air or cooling water.

Partially or fully condensed refrigerant in line 328 is further cooledin heat exchanger 357 to provide cooled refrigerant in line 369 and thisrefrigerant stream is divided into first and second portions. The firstportion is reduced in pressure across throttling valve 371 and thereduced-pressure refrigerant, which may be defined as an auxiliaryrefrigerant, flows via line 373 to heat exchanger 357, where it iswarmed and vaporized to provide refrigeration therein and to yield thevaporized auxiliary refrigerant stream in line 367. The second portionof the cooled refrigerant flows via line 329 and is further cooled inheat exchanger 312 to yield refrigerant 313 described above. Thus theauxiliary refrigerant in lines 367 and 373 is derived from therefrigerant in line 315 and in this embodiment has the same compositionas the refrigerant in line 315.

Optionally, separator 321, pump 325, compressor 324, and cooler 327 arenot used and the partially or fully condensed refrigerant in line 328 isprovided directly from cooler 320.

Typically, the low pressure refrigerant stream in line 315 will bevaporized in a pressure range of about 2 to 10 bara in heat exchanger312, while intermediate pressure refrigerant stream in line 373 will bevaporized at a higher pressure in a range of about 5 to 20 bara in heatexchanger 357. When the above embodiments are used for the liquefactionof natural gas, hydrocarbons heavier than methane may be condensed andremoved before final methane liquefaction by known methods includingscrub columns or other partial condensation and/or distillationprocesses. As described above, these condensed natural gas liquids(NGLs) may be fractionated to provide selected components for therefrigerants in the refrigeration systems. Modifications to thisembodiment may include provision for vaporizing a portion of therefrigerant in line 369 at a third higher pressure to provide warmerrefrigeration than that provided by the lower pressure vaporizingrefrigerant streams in lines 315 and 373.

Another alternative to the exemplary embodiment of FIG. 2 is illustratedin FIG. 7. In this exemplary embodiment, the third refrigeration loopthat provides refrigeration to cold heat exchanger 312 is modified toprovide autorefrigeration by an internal auxiliary liquid refrigerantderived from the third refrigerant by phase separation. In theembodiment of FIG. 7, the feed stream in line 9 being liquefied iscooled to its final lowest temperature in this cold heat exchanger andreduced in pressure to provide liquid product in line 13, which may bereduced in pressure to provide a reduced-pressure product in line 17.The refrigeration for this final cooling is provided by cooling therefrigerant in line 329 to provide cooled liquid refrigerant in line313, reducing the pressure across throttling valve 314 to yieldreduced-pressure refrigerant in line 315, and partially or fullyvaporizing this refrigerant to provide the refrigeration in heatexchanger 312.

The partially or fully vaporized refrigerant in line 316 is combinedwith a reduced-pressure refrigerant provided by pressure reductionacross throttling valve 375 (described below) to yield a combinedrefrigerant stream in line 377. This combined refrigerant stream, whichmay be described as an auxiliary refrigerant stream, is warmed andvaporized in heat exchanger 379 to provide refrigeration therein and togenerate vaporized auxiliary refrigerant in line 381. This vaporizedauxiliary refrigerant is compressed in compressor 319, cooled andpartially condensed in intercooler 320, and separated in separator 321to provide a vapor stream in line 322 and a liquid stream in line 323.

The vapor stream in line 322 is further compressed to a pressure of 30to 70 bara in compressor 324, the liquid stream from separator 321 ispressurized by pump 325 to the same pressure, the two pressurizedstreams are combined to provide two-phase refrigerant stream 326, whichis further cooled in aftercooler 327 by air or cooling water to providea partially condensed auxiliary refrigerant in line 328.

Optionally, separator 321, pump 325, compressor 324, and cooler 327 arenot used and the partially condensed auxiliary refrigerant in line 328is provided directly from cooler 320.

This partially condensed auxiliary refrigerant flows via line 328 toseparator 330, where it is separated to yield a vapor refrigerantfraction in line 385 and a liquid refrigerant fraction in line 383. Theliquid refrigerant fraction in line 383 is cooled in heat exchanger 379to yield cooled refrigerant in line 389, which is reduced in pressureacross throttling valve 375 and combined with partially or fullyvaporized refrigerant in line 316 to yield the auxiliary refrigerantstream in line 377.

Thus the refrigerant in lines 328, 377, and 381 is an auxiliaryrefrigerant derived from the refrigerant in line 315. In thisembodiment, this auxiliary refrigerant contains the same components buthas a different composition than the refrigerant in line 315. Thedifferent composition is a result of the phase separation of thepartially condensed refrigerant in line 328 to yield the liquidrefrigerant in line 387 and the vapor refrigerant in line 385.

Both embodiments illustrated in FIGS. 6 and 7 differ from the prior artin that refrigerant from the third cooling loop that provides the lowesttemperature of refrigeration is vaporized to provide refrigeration intemperature ranges both above and below the lowest temperature of thefeed being cooled by the second cooling loop in heat exchanger 311. Thusrefrigeration above the lowest temperature of the feed in heat exchanger311 is provided in heat exchanger 357 (FIG. 6) or 379 (FIG. 7) byvaporizing an auxiliary refrigerant derived from the refrigerant in line315 that provides the refrigeration in coldest heat exchanger 312.Refrigeration below the lowest temperature of the feed being cooled inheat exchanger 311 is provided in heat exchanger 312. Thus therefrigeration in these two temperature ranges, i.e., both above andbelow the lowest temperature of the feed in heat exchanger 311, isprovided by the third cooling loop.

Additionally, all or most of the refrigeration to cool the high pressurerefrigerants in lines 383 and 385 after phase separation is provided byautorefrigeration in the third cooling loop, and this cooling of thecompressed refrigerant in the third cooling loop is performedindependently of the two warmer cooling loops. This feature isbeneficial in that it greatly unloads the refrigeration requirements ofthe two warmer cooling loops, thereby allowing larger product capacitiesin natural gas liquefaction plants when the compressors in the twowarmer cooling loops reach the maximum commercially available size.Either of the embodiments of FIGS. 6 and 7 may be installed as aretrofit to an existing two-loop dual mixed refrigerant liquefactionplant or two-loop propane-mixed refrigerant natural gas liquefactionplant.

EXAMPLE

The process of FIG. 3 is illustrated by the following non-limitingexample in which a feed gas stream of 100 kg-moles/hr of natural gas inline 1 is liquefied to provide a liquefied natural gas (LNG) product inline 17. The feed gas in line 1, having been purified previously (notshown) to remove water and acid gas impurities, is provided at atemperature of 27° C. and a pressure of 60 bara. The feed gas in line 1and mixed refrigerant vapor in line 207 are cooled to a temperature of−33° C. in first heat exchanger 129 by three stages of propane cooling.To effect this cooling, propane is evaporated at three pressure levelsforming three suction streams (135,145, and 151) to propane compressor119. The pressures of the three suction streams are 1.3 bara, 2.8 bara,and 4.8 bara respectively. Compressor 119 has a discharge pressure of16.3 bara. The propane is cooled to a temperature of 43° C. andcondensed in aftercooler 121 using an ambient temperature cooling mediumsuch as cooling water or air. Total propane flow in line 123 is 114kg-mole/hr.

The cooled feed in line 5 and second mixed refrigerant in line 208 arecooled to a temperature of −119° C. in second heat exchanger 311 toyield further cooled feed in line 9 and further cooled second mixedrefrigerant in line 209. The mixed refrigerant in line 209 is throttledacross valve 211 to a pressure of 4.2 bara to yield a reduced-pressuremixed refrigerant in line 213. The mixed refrigerant in line 213 isvaporized in heat exchanger 311 to provide refrigeration therein. Themixed refrigerant for this second cooling loop has a flow rate of 87 kgmoles/hr and a composition of 27 mole % methane, 63 mole % ethane and 10mole % propane. The vaporized second mixed refrigerant stream in line201 is compressed in three-stage intercooled compressor 203 to apressure of 57 bara. The compressed mixed refrigerant is cooled inaftercooler 205 to 36.5° C. using cooling water to provide cooledcompressed mixed refrigerant in line 207.

Feed in line 9 and third mixed refrigerant in line 329 are cooled to afinal temperature of −156° C. in third heat exchanger 312 to yield,respectively, liquefied LNG product in line 17 and condensed third mixedrefrigerant in line 313. The mixed refrigerant in line 313 is throttledacross valve 314 to a pressure of 3.7 bara to provide reduced-pressurethird mixed refrigerant in line 315. This reduced-pressure third mixedrefrigerant partially vaporizes in third heat exchanger 312 to providerefrigeration therein, and the partially-vaporized refrigerant in line316 has a vapor fraction of 55% and a temperature of −123° C. The mixedrefrigerant for this third cooling loop has a flow rate of 59kg-moles/hr and a composition (in mole %) of 12% nitrogen, 52% methane,18% ethane, 6% propane, and 12% i-pentane.

Mixed refrigerant in line 316 is fully vaporized and warmed to 26° C. infourth heat exchanger 317 to provide refrigeration therein. Vaporizedrefrigerant in line 318 is compressed to 17.7 bara in first stagecompressor 319, cooled to 36.5° C. and partially liquefied inwater-cooled intercooler 320. The two-phase refrigerant is separated inseparator 321 to yield refrigerant vapor in line 322 and refrigerantliquid in line 323. The refrigerant liquid is pressurized in pump 325 to47 bara. Refrigerant vapor in line 322 is compressed to a pressure of 47bara in compressor 324, combined with the pressurized refrigerant frompump 325, and the combined stream in line 326 is cooled in water-cooledaftercooler 327 to 36.5° C. to yield cooled mixed refrigerant in line328. This mixed refrigerant is cooled in fourth heat exchanger 317 toprovide cooled mixed refrigerant in line 329, which is further cooled inthird heat exchanger 312 as earlier described.

In the above description of FIGS. 1-7, reference numbers for lines(i.e., pipes through which process streams flow) also may refer to theprocess streams flowing in those lines. In the following method claims,reference numbers denote process streams flowing in those lines. In thefollowing system claims, the reference numbers denote the lines ratherthan the process streams flowing in these lines. Reference numbers fromFIGS. 2-7 are included in the following claims for clarity and are notmeant to limit the scope of the claims in any way.

1. A method for liquefying a gas (1) which comprises cooling a feed gasstream successively through at least two heat exchange zones (310, 311,312; 353) at respective temperature ranges to provide a liquefiedproduct (13), wherein refrigeration for cooling the feed gas stream inthe temperature ranges is provided by respective vaporizing refrigerants(117, 213, 315), wherein the refrigerant (315) in the coldesttemperature range is only partially vaporized in the coldest heatexchange zone (312) to form a partially vaporized refrigerant (316), andwherein the refrigerant is recirculated in a recirculating refrigerationprocess that comprises further vaporizing the partially vaporizedrefrigerant (316) in a further heat exchange zone (317, 355) attemperatures above the highest temperature of the coldest heat exchangezone (312) to form a totally vaporized refrigerant (318, 348),compressing (319, 324; 349) the totally vaporized refrigerant (318, 348)to yield a compressed refrigerant stream, and cooling the compressedrefrigerant stream to provide a coldest refrigerant (315), characterizedin that the entire compressed refrigerant stream is cooled by either (i)cooling the entire compressed refrigerant stream (328) in the furtherheat exchange zone (317) by indirect heat exchange with the furthervaporizing partially vaporized refrigerant (316) to provide a cooledrefrigerant stream (329), thereby providing self-refrigeration for therecirculating refrigeration process, and then by further cooling (312)the cooled refrigerant stream (329) to provide the coldest refrigerant(315), or (ii) cooling the entire compressed refrigerant stream (351) ina heat exchange zone (353) preceding the coldest heat exchange zone(312) by indirect heat exchange (352) with a respective vaporizingrefrigerant (117), further cooling the refrigerant in the further heatexchange zone (355) by indirect heat exchange with the partiallyvaporized refrigerant (316) to provide a cooled refrigerant stream(329), and then further cooling (312) the cooled refrigerant stream(329) to provide the coldest refrigerant (315).
 2. A method of claim 1wherein the feed gas stream (1) is natural gas.
 3. A method of claim 1or claim 2 wherein the refrigerant (315, 316, 318, 328, 329) in therecirculating refrigeration process is a multicomponent mixturecomprising nitrogen, i-pentane, and n-pentane with the molar ratio ofi-pentane to n-pentane in the refrigerant (315, 316, 318, 328, 329)being greater than one, and wherein the i-pentane and n-pentane areobtained from the feed gas stream (1) and the molar ratio of i-pentaneto n-pentane in the refrigerant (315, 316, 318, 328, 329) is greaterthan the molar ratio of i-pentane to n-pentane in the feed gas stream(1).
 4. A method of claim 1 or claim 2, wherein the refrigerant (315,316, 318, 328, 329) in the recirculating refrigeration process is amulticomponent mixture comprising nitrogen, i-pentane, and one or morehydrocarbons having four carbon atoms, the i-pentane and the one or morehydrocarbons having four carbon atoms being obtained from the feed gasstream (1), and the molar ratio of i-pentane to n-pentane in therefrigerant (315, 316, 318, 328, 329) being greater than one, whereinthe i-pentane and n-pentane are obtained from the feed gas stream (1)and the molar ratio of i-pentane to the one or more hydrocarbons havingfour carbon atoms in the refrigerant (315, 316, 318, 328, 329) isgreater than the molar ratio of i-pentane to the one or morehydrocarbons having four carbon atoms in the feed gas stream (1).
 5. Amethod of claim 1 or claim 2 wherein the refrigerant (315, 316, 318,328, 329) in the recirculating refrigeration process comprises (in mole%) 5-15% nitrogen, 30-60% methane, 10-30% ethane, 0-10% propane, and5-15% i-pentane.
 6. A method of claim 1, wherein the further cooling ofthe cooled refrigerant stream (329) is effected by indirect heatexchange with the coldest refrigerant (315) vaporizing in the coldestheat exchange zone (312).
 7. A method of claim 1, wherein, prior tovaporization to cool the compressed refrigerant stream (328, 339), acooled reduced-pressure liquid refrigerant (345) is reduced in pressureand is combined with the partially vaporized refrigerant (316) toprovide a combined two-phase refrigerant (347) that is vaporized to coolthe compressed refrigerant stream (328; 339), the compressed refrigerantvapor (330) is cooled (332) to provide a partially-condensedrefrigerant, the partially condensed refrigerant is separated (333) intoa refrigerant vapor stream (334) and a refrigerant liquid stream (335),the refrigerated vapor stream (334) is compressed (336) and cooled (337)to form a partially condensed stream and the partially-condensed streamis separated (338) into the compressed refrigerant vapor (328, 339) anda liquid stream (340), the pressure of the liquid refrigerant is reduced(341) to provide a reduced-pressure liquid refrigerant (342), thereduced-pressure liquid refrigerant (342) is combined with therefrigerant liquid stream (343) and subcooled by indirect heat exchange(344) with the combined two-phase refrigerant (347) to provide thecooled reduced-pressure liquid refrigerant (345) for combining with thepartially vaporized refrigerant (316).
 8. A system for liquefying a gasstream (1) by a method of claim 1, which system comprises: at least twoheat exchange zones (310, 311 312; 353) adapted for cooling the gasstream (1) successively through respective temperature ranges to providea liquefied product (13) and respective refrigeration systems forproviding respective refrigerants in respective refrigerant lines (117,213, 315) to the heat exchange zones (310, 311, 312; 353), wherein thecoldest heat exchange zone (312) is adapted to only partially vaporizethe respective (i.e., coldest) refrigerant (315), wherein therefrigeration system providing the coldest refrigerant is arecirculating refrigeration system comprising: a further heat exchangezone (317, 355) adapted to further vaporize the resultant partiallyvaporized refrigerant at temperatures above the highest temperature ofthe coldest heat exchange zone (312), compression means (319, 324; 349)for compressing the vaporized refrigerant to provide the compressedrefrigerant stream, piping means (318, 348) to provide vaporizedrefrigerant from the further heat exchange zone (317, 355) to thecompression means (319, 324; 349), means to provide compressedrefrigerant (328, 354) to the further heat exchange zone (317, 355),piping means to convey a cooled compressed refrigerant from the furtherheat exchange zone (317; 355) to the coldest heat exchange zone (312),and means to further cool (356) the cooled compressed refrigerant toprovide a cooled compressed refrigerant (313), characterized in that themeans to provide compressed refrigerant (328; 354) to the further heatexchange zone (317, 355) comprises either (i) piping means (329) toconvey the entire compressed refrigerant stream (328) to the furtherheat exchange zone (317), wherein the further heat exchange zone (317)is adapted to provide self-refrigeration for the recirculatingrefrigeration system, or (ii) piping means to convey the entirecompressed refrigerant stream (351) to a heat exchange zone (353) thatprecedes the coldest heat exchange zone (312), wherein the heat exchangezone (353) is adapted to cool the entire compressed refrigerant stream(351) by indirect heat exchange (352), and piping means to convey anintermediate cooled compressed refrigerant (354) to the further heatexchange zone (355).
 9. A system of claim 8, wherein the means tofurther cool (356) the cooled compressed refrigerant to provide acondensed refrigerant comprises the coldest heat exchange zone (312) andthe system further comprises pressure reduction means (314) to reducethe pressure of the condensed refrigerant to provide the refrigerant tothe refrigerant line (315) for the coldest heat exchange zone (312). 10.A system of claim 9, wherein the further heat exchange zone (344)includes means for subcooling a refrigerant liquid to provide asubcooled refrigerant liquid and the coldest refrigeration systemcomprises pressure reduction means (346) for reducing the pressure ofthe subcooled refrigerant liquid to provide a reduced-pressurerefrigerant, piping means (347) for combining the reduced-pressurerefrigerant with the partially vaporized refrigerant from the coldestheat exchange zone (312) to provide a combined vaporizing refrigerantstream to the further heat exchange zone (344), and piping means (330)to feed a combined vaporized refrigerant stream to the compressionmeans.
 11. A system of claim 10, wherein the compression means forcompressing the vaporized refrigerant (330) from the further heatexchange zone (344) comprises: a first stage compressor (331), anintercooler (332) adapted to cool and partially condense the resultantfirst compressed refrigerant stream from first stage compressor (331) toyield a partially-condensed first refrigerant stream, a first separator(333) to separate the partially condensed first compressed refrigerantstream into a first vapor refrigerant stream and a first liquidrefrigerant stream, a second stage compressor (336) to compress thefirst vapor refrigerant stream to provide a compressed vapor refrigerantstream, an aftercooler (337) to cool the compressed vapor refrigerantstream to provide a cooled two-phase refrigerant stream, a secondseparator (338) to provide a second liquid refrigerant stream (340) andthe and the compressed refrigerant to piping means (339) for feed to thefurther heat exchange zone (344), pressure reduction means (341) toreduce the pressure of the second liquid refrigerant stream to provide areduced-pressure second refrigerant stream, and piping means (335, 342,343) to combine the reduced-pressure second refrigerant stream and thefirst liquid refrigerant stream to provide the refrigerant liquid to thefurther heat exchange zone (344).
 12. A system of claim 8 for liquefyinga gas stream (1) by a method of claim 1, wherein the coldestrefrigeration system comprises: piping means (348) to provide thevaporized refrigerant from the further heat exchange zone (355) to thecompression means (349) for compressing a vaporized third refrigerant(348) to provide a compressed refrigerant, cooling means (352) in a heatexchange zone preceding the coldest heat exchange zone (312) for coolingthe compressed refrigerant (351) by indirect heat exchange with therespective refrigerant (117) vaporizing in the heat exchange zone (353)to provide a cooled compressed refrigerant, piping means (354) toprovide the cooled compressed refrigerant to the further heat exchangezone (355) to further cool the cooled compressed refrigerant by indirectheat exchange with the vaporizing refrigerant from the coldest heatexchange zone (312) to provide a condensed refrigerant (329) and thevaporized third refrigerant (348), and pressure reduction means (314) toreduce the pressure of the condensed refrigerant to provide therefrigerant to the refrigerant line (315) to the coldest refrigerantzone (312).